A tile or other ceramic products are obtained by firing materials in a kiln with or without glaze.
The products thus obtained may vary in color depending on atmosphere in which the materials are fired. A tasteful color and appearance may from time to time be obtained when the materials are fired, particularly in a reducing atmosphere.
To this end, a reducing atmosphere is conventionally created in a kiln to simultaneously fire and reduce materials.
Japanese patent publication No. 17797/90 discloses a continuous kiln which includes a preheating zone, a firing zone and a cooling zone. Materials are continuously conveyed through the kiln. A reducing burner is mounted so as to apply a reducing flame directly to the material during a reducing operation, rather than a firing operation.
A roller hurse kiln, as a continuous kiln, includes a tunnel furnace, a number of rollers by which a material to be fired is conveyed through the furnace, a burner mounted to the furnace, and a fan assembly. Each horizontal roller has an axis extending at right angles to the longitudinal direction of the furnace and is rotated by a motor. The rollers are spaced for a short distance away from one another. Materials to be fired are continuously conveyed on the rollers. The furnace has a preheating zone at its inlet, a firing zone at its center, and a cooling zone extending from the center to its outlet.
In the continuous kiln of Japanese patent publication No. 17797/90, a flame is applied from the reducing burner directly to materials during a reducing operation rather than a firing operation. An external gas is excluded to maintain a reducing atmosphere. This results in uniform reduction of the materials as conveyed continuously through the furnace to continuously provide products of equal color and constant quality on a mass production basis.
A reducing atmosphere is produced generally by maintaining an air-to-fuel ratio for the firing burner below its normal ratio. However, it is difficult to maintain and control reducing conditions under such a reducing atmosphere for the reasons as will later be explained.
Reduction is to remove oxygen from a material as fired. When fuel in gas phase (for example, propane gas) including carbon and hydrogen is used, a mixture of carbon monoxide and hydrogen, produced as a decomposition by a burner flame, is used as a reducing gas to reduce the material. Carbon monoxide contains a substantial amount of carbon and is stable at a high temperature or above 1200.degree. C. Carbon monoxide is however not stable at a temperature below 1200.degree. C. If oxygen is contained slightly more than it should be, carbon monoxide is decomposed to carbon and carbon dioxide as a result of carbon deposition reaction as its temperature decreases.
When a material is reduced under a reducing atmosphere, it is difficult to completely eliminate external atmosphere such as those in the preceding and following operations. It is difficult to maintain an atmosphere suitable for reducing a material.
The temperature of a material during a reducing operation is also an important factor. It is extremely difficult to uniformly reduce materials under such an atmosphere while adjusting the temperature of the materials. A conventional method for reducing materials as fired is unable to constantly provide products of a desired reduced color.